Most battery stack monitors (BSM) directly connect to a battery stack and can only measure individual battery cell voltages. Passive balancing via MOSFETs and resistors are typically used to balance cell voltage and/or cell stored charge. Cell balancing may have to be turned off during battery cell voltage measurement by the BSM; otherwise, significant reading errors may be introduced. This may be due to the IR drop in cable-impedances, interconnect-impedances, cell impedance, and internal battery ESR. Due to these IR drops, cell balancing duty cycle may be reduced or in some cases balancing may not be possible. There are no known alternative methods for compensating IR drops among the battery equivalent series resistance (ESR), cable wire, and connectors for calculating accurate battery float voltage during active or passive balancing. There are methods in the industry for calculating float voltage by compensating for battery ESR during charging and discharging the entire battery stack, but do not apply to balancing during battery cell measurement.
Hence a need exists to enable battery cell voltage measurement by BSM during balancing and overcome measurement error during balancing (e.g., error compensation to cell float voltage), which allows a balancing circuit to safely operate during charge and discharge of the cells, increase balancing duty cycle, and optimizes size and cost of the balancing circuit.